Safe-arm mechanism for explosive trains



Jan. 31, 1967 J. HENNESSY ETAL 3,301,183

SAFE-ARM MECHANISM FOR EXPLOSIVE TRAINS Filed June 30, 1965 FIG-2 4 INVENTORS GEORGE A. NODDIN JAMES HENNESSY ATTORNEY United States Patent C i 3,301,183 SAFE-ARM MECHANISM FOR EXPLGSIVE TRAINS James Hennessy, Pittman, and George A. N oddin, Mantua,

NJ., assignors to E. I. du Pont de Nemours and OH1- parry, Wilmington, Del., a corporation of Delaware Filed June 30, 1965, Scr. No. 468,425 7 Claims. (Cl. 1tl227) This invention relates to arming systems for explosive trains and, more particularly, to arming systems which are disarmed in a gaseous medium such as air but armed when surrounded by a liquid medium such as water.

For activities in which the detonation of explosive charges below the surface of water is desired, such as in seismic exploration, underwater signaling, echo-ranging for detection purposes, and destruction of undersea craft, there is a need for a reliable and accurate explosive train which can be armed, i.e., made capable of propagating an initiation impulse, when submerged, yet will be safe, i.e., disarmed, by virtue of being incapable of propagating a detonation stimulus under conditions of storage or removal from the water or other liquid.

Commonly, mechanisms for precluding accidental actuation of explosive trains comprising a donor charge, e.g., an initiator, and an acceptor charge in explosive assemblies intended for underwater use depend upon physical separation of the components by maintaining these elements out of line or separated by a gap sutficient to preclude actuation of the acceptor charge, should accidental actuation of the donor charge occur. To arm such an explosive train, mechanisms are provided to move the components into position, i.e., nearer to one another or into line, so that continuous detonation of the explosive train will occur upon actuation of the donor charge. This type of arming mechanism generally requires the use of complex mechanical systems which, in order to function properly, must be in prescribed alignment and adjusted to move the elements smoothly from the safe to the armed positions. However, the requisite alignment and adjustment are difficult to insure since the mechanisms are susceptible to damage during handling and launching of the explosive assemblies containing them. Further, arming of these mechanisms is often irreversible, thus once armed, such assemblies must be fired.

Recently, an arming system has been proposed wherein the donor charge and the acceptor charge in an explosive train are separated by a fixed distance sufficient to preelude actuation of the acceptor charge by the donor charge when the two are in air but to allow propagation of a detonation impulse from the donor to the acceptor charge when the two are in a liquid, i.e., in water. The train is armed by changing the medium in the space separating the donor and acceptor charges, i.e., the train is armed by replacing the air, normally occupying this space in the assembly before the train is armed, by a liquid, i.e., water, by immersing the train in the liquid or by pumping liquid into the space. This system which is described in US. 3,065,694, is based on the theory that shock waves are conducted more efficiently by a liquid, e.g., water, than by a gaseous medium, e.g., air. Although such a system is attractive in that it would minimize or elimi nate the need for complex arming mechanisms, the theory upon which the system is based does not prove valid or practicable for commercially available explosive charges. Rather it has been. determined experimentally. using com mercially available explosive charges and accessories, that a detonation impulse is not propagated as readily between spaced apart donor and acceptor charges when the train is in a liquid, i.e., ,water, as when the train is in air. For example, when a 4-inch long piece of detonating cord containing a central core of lead azide at a loading of 34 3 ,301,183 Patented Jan. 31, 1967 grains per foot is used as the donor charge and the acceptor charge is 4 grains of PETN pressed into a thin bottomed tubular aluminum cap shell 0.240 in. OD, a spacing of at least '7 inch must be provided to preclude actuation of the acceptor charge by the donor charge when the two are in air. However, when water occupies the space between the charges, the detonation impulse cannot be propagated from the donor charge to the acceptor charge over this distance. In fact, when the space is occupied by water, the spacing must be inch or less to allow propagation from the donor to the acceptor charge Thus, an arming system in which the donor and acceptor charges of an explosive train are separated by a distance sufiicient to preclude propagation of a detonation impulse in air would still preclude propagation when the charges are in water, i.e., the train would not be armed in Water.

Accordingly, a need still exists for an arming system for underwater use in which the arming mechanism is not dependent upon the interaction of mechanical parts but is positive and reliable in functioning. Such a system should be reliably operational at both shallow and great depths and should be easily incorporated into the explosive train.

In accordance with this invention, it has now been found that a detonation stimulus can be propagated reliably and emciently by a length of low-energy detonating cord having a segment thereof formed into a helix, i.e., a helical coil, when this segment is surrounded by a liquid, e.g., water; however, when such a segment is surrounded by a less dense medium, i.e., a gaseous medium such as air, a detonation stimulus transmitted to the cord is cut off within the helical segment.

Accordingly, the present invention provides a safe-arm mechanism comprising a helix of 3 to 10 tightly coiled turns of low-energy connecting cord having a core of high velocity, cap-sensitive detonating explosive at a loading of about 1.5 to 2.2 grains per foot encased in a ductile metal sheath, said helix being characterized by its ability to propagate a detonation stimulus when surrounded by a liquid medium and its inability to propagate such a stimulus when surrounded by a gaseous medium.

This invention also provides an arming system wherein a length of low-energy connecting cord containing an exposed helix as described above connects a donor charge (e.g., an initiator) to an acceptor charge (e.g., a base, booster or primer charge) when is separated from the donor charge at least a distance suflicient to preclude sympathetic actuation of the acceptor charge by the donor charge. The helix will propagate a detonation stimulus emanating from the donor charge to the acceptor charge as long as it is surrounded by (i.e., immersed in) liquid, e.g., water or oil, and, accordingly, the train of charges is positively armed. However, when a gaseous medium such as air surrounds the helix, a detonation impulse emanating from the donor charge is cut off within the helix and is not imparted to the acceptor charge. In other words, the explosive charge is disarmed, i.e., safe, in air since actuation of the acceptor charge by the donor charge is precluded. The safe-arm mechanism of this invention is particularly preferred for arming explosive trains designed for underwater use and which must be safe before being submerged.

The expression exposed helix is used to denote that the helix is neither peripherally nor centrally enclosed or confined in such a manner that it will be incapable of being contacted and completely surrounded by the arming liquid medium. As used herein, the expression tightly coiled turns means that the adjacent turns are in contact or closely adjacent to eath other.

In order to describe the invention in greater detail, reference is now made to the accompanying drawings wherein: t

FIGURE 1 is a sectional view of an arming system constructed in accordance with this invention;

FIGURE 2 is a sectional view of an alternative embodiment of an arming system constructed in accordance with this invention; and

FIGURE 3 is an end view of a preferred embodiment of the safe arm mechanism of this invention.

Referring now to the drawings, in which like reference characters designate like or corresponding parts,

In FIGURE 1 there is shown an initiator designed for underwater use having a protective shell or housing 1 (e.g., of metal or plastic) which contains base (acceptor) charge 2, and primary detonating composition (donor charge) 3. Extending between charges 2 and 3 is a helically-coiled length, i.e., a helix, of low-energy connecting cord 4, comprising a central core 5 of high velocity detonating explosive at a core loading of about from 1.5 to 2.2 grains/foot in a sheathing of a ductile metal. The ends of the cord extend through and are held in propagating relationship to the donor and acceptor charges, respectively, by grommets 6 which also act as battles that prevent liquid from desensitizing such charges. The donor charge 3 is confined on the bore of heavywalled tube 7 thus permitting a small quantity thereof to initiate detonation in the core of the cord. Ignition composition 8 is in propagating relationship to charge 3 and actuation of the ignition charge is brought about by heating of bridgewire 9 which spans leadwires 10 extending into the shell through closure plug 11. The portion of shell 1 surrounding the helix is of larger diameter than the helix and is provided with perforations of orifices 12 to allow flow of fiuid into contact with and around the helix. The location, distribution, and size of the perforations are selected to permit any desired rate of flow of fluid into the chamber containing the helix so that the chamber will be filled when actuation of the initiator is to occur, e.g., at a predetermined depth in water.

In FIGURE 2, there is shown a portion of a device designed for underwater use having a cartridge 13 which houses a main charge 14. This cartridge may be of any desired configuration, but here is shown as cylindrical. Shell 1, which contains base or booster charge 2, fits into the main charge so that the booster charge is at least partially surrounded by the main charge. In this embodiment donor charge 3 is fired in response to the hydro static pressure at a predetermined depth. In the pressure-actuated assembly shown, 15 is an anvil and positioned on top of and extending through the anvil is an ignition charge 16 of a percussion-sensitive explosive. Integrally formed at one edge of anvil 15 is a striker-arm arrester 17 for striker arm 18 attached to piston 19 which is slidably mounted in the open end of the initiator shell.

The explosive train shown in FIGURES 1 and 2 is in an inert or safe condition during normal storage and handling due to the fact that the helically coiled length of low-energy connecting cord is surrounded by air and a detonation impulse transferred to the cord by the donor charge will be cut off before it is transferred to the acceptor charge. The reason for this attenuation of the detonation stimulus is unknown, however, the cord in the helix does not rupture in air and thus disconnect the explosive train. The spacing between the donor and the acceptor charges is normally chosen in accordance with the air gap-sensitivity of the acceptor charge, i.e., the spacing must be sufiicient that actuation of the initiator does not effect sympathetic initiation of the acceptor when the two are in air. If another gaseous medium is employed, the sensitivity of the acceptor charge in that medium will determine the spacing. When the device is submerged in liquid, e.g., water or another dense liquid medium such as oil, the perforations or orifices allow entrance of the liquid into the chamber containing the helix either directly as in the figures or indirectly, e.g., after the liquid first dissolves soluble disks which may .cover the perforations. In time, liquid fills the chamber and completely surrounds the helix of low-energy connecting cord. Actuation of the donor charge is effected by applying a firing current to the lead wires for an electric system as shown in FIGURE 1. When a pressure-actuated initiator is used as in FEGURE 2, actuation is effected by inward movement of the piston 19 under hydrostatic pressure such that the striker arm 18 is bent to a degree that the free end slips off arrester 17 and snaps onto ignition charge 16. The impulse produced upon actuation of the donor charge 3 actuates the explosive cord, which transmits the detonation impulse with out hiatus to the acceptor or base charge 2 which may, in turn, initiate a main charge 14. Actuation of the electric assembly may be my means, e.g., a sea battery, a pull-cord actuated switch, a pressure-actuated fuze, etc., adapted to fire the donor charge at the predetermined depth.

FIGURE 3 shows a preferred embodiment of the helix of this invention which further insures that the detonation stimulus will be cut off in a gaseous medium. In this figure which is a view of the end of the helix to be con nected to the acceptor charge, segment 4A of the cord 4 extends across the end of the opening through the center of the helix before turning downward toward the acceptor charge. When the detonation impulse from the donor charge starts through the helix, some of the sheathing metal is jetted downward through the center of the helix, and this jet will cut segment 4A of cord provided it extends at least partially across the central opening through the helix and hence the jets path. Thus, should the detonation stimulus not be cut oil in the coils of the helix. it will stop when it reaches the severed end of the cord.

The low-energy explosive-connecting cord employed in the safe-arm mechanism of this invention comprises a core of a high velocity O meters/second) cap-sensitive detonating explosive at a core loading of about 1.5 to about 2.2 grains per foot contained in an uncountered sheath of ductile metal, preferably lead. Suitable highvelocity detonating explosives for use in the cord include PETN, RDX, HMX, bis(trinitroethyl)urea, tetranitrodibenZ0-l,3a,4,6a(or -l,3a,6,6a)-tetraazapentalene, and the like, and mixtures of two or more of these explosives. Such explosives can be used alone or in conjunction with conventional additives, e.g., graining agents and binders, provided the actual explosive loading is as described above. Lead-sheathed cords which are prepared by drawing or combined drawing and swaging usually provide better safe-arm systems for use underwater than those prepared by swaging alone.

The cord is wound into a tight helix, with the ductile metal sheath of adjacent turns preferably being in contact. The cord is self-supporting sothat there is no need for supports to maintain the tight configuration; in fact such supports usually detrimentally effect the safe-arm characteristics of the explosive train.

The ability of the helix to cut off the detonation im pulse partially depends on the thickness of the ductile metal sheath (inches) relative to the core loading (grains/ foot). A ratio of core loading to sheathing thickness of about from :1 to 220:1 will usually be employed and such cord is commercially available as uncountered LEDC. The lower ratios are employed with more powerful, and more easily initiated cores.

As a rule, the helix will have from 3 to 10 turns of the cord. Generally, when only one or two turns are employed, cut-off of the detonation stimulus is not reliably achieved, whereas more than 10 turns of the cord will not reliably propagate under water. Particularly good safe-arm properties are provided by coiling cord having a core of about 1.8 gr./ft. PETN encased in 0.040 to 0.045 inch-outer diameter lead sheathing, into 3 to 5 tight turns having an inside diameter of about (.141) inch. This helix reliably propagates a detonation stimulus when submerged in water yet the detonation stimulus will reliably be cut oil when in air. The helix will usually be formed by wrapping the cord around a mandrel whose diameter is such that the internal diameter of the helix is about from 1.75 to 8 times the diameter of the cord. The mandrelmust be removed before the helix is placed in the ex plosive train, and after its removal, the helix preferably will be provided with a segment of 4A (FIGURE 3) by bending the end of the cord at least part way across the central opening left by the mandrel.

As stated, the distance between the donor charge and the acceptor charge must be such that the initiation impulse produced by the donor charge will not initiate the acceptor charge when the system is in the gaseous medium, generally air. For some applications it may be desirable to make this spacing 'sufiicient to preclude sympathetic actuation of either charge by the other. Grommets 6 of a rubbery or elastomeric composition between the acceptor and donor charges act as baffles and help to minirnize the spacing required to insure that the acceptor charge is not initiated in sympathy with the actuation of the donor charge. For example, in initiatorsas shown in the figures, the spacing between donor and acceptor charges must be at least inch and preferably is at least one inch When the donor charge is 0.7 grain of lead 'azide and the acceptor charge is PETN, PETN being one of the most sensitive of the acceptor charges contemplated for use. Accordingly spacing of this magnitude should preclude sympathetic actuation of other acceptor charges such as of RDX, HMX, pentolite, and the like. Greater spacing will, of course, be required with larger charges of any given explosive and with more powerful charges.

A variety of ignition assemblies may be used with the arming mechanism and system of this invention. A pressure-actuated assembly is particularly suitable since reliable initiation and actuation of the explosive train will occur only in a liquid at a predetermined depth, i.e., pressure. However, the initiation may be by a conventional electric ignition assembly or electric delay ignition assembly fired by a bridgewire in a loose ignition composition, a bridgewire and bead arrangement, an exploding bridgewire, or arc-firing system, in which cases the lead wires extend to a source of electric current at the surface of the water and the sink rate of the system must be known. Alternatively the initiator could be fired by a water-actuated battery directly or through a pressure-sensitive switch which will fire the system at the predetermined depth.

If desired, the explosive train can be provided with a reservoir for fluid (water) and a conventional two-way pump, which may be actuated by a timing mechanism, to pump water to and from a chamber surounding the helix of low-energy connecting cord.

The invention is illustrated by the following example.

Example Ten units similar to that shown in FIGURE 1 are prepared. The housing is of 0.010 in. thick aluminum and has an outer diameter (O.D.) of 0.280 inch and an inner diameter (I.D.) of 0.260 inch. Five grains of superfine PETN as the acceptor charge is placed in the base of the housing and pressed into place. A grommet /2 inch long and 0.255 inch in diameter and having a central aperture is placed above the primer charge to protect the primer charge from water and to aid in retaining the length of low-energy connecting cord. A 2 inch length of drawn low-energy connecting cord having a core loading of 1.8 gr./ft. of PETN in a 0.040 inch O.D. lead sheath is coiled to form a helix inch LD. composed of 4 turns, with adjacent turns being in contact and one end of the cord extending diametrically across the base of the helix and then downward into the grommet. The free end of the cord is pushed through a rubber grommet A inch long and 0.260 inch in diameter and terminates in 6 propagating relationship to a donor charge of lead azide which is contained in the bore of a 0.25 inch long lead tube (the core loading of the lead az'ide is 34 gr./ft.). The portion of the housing surrounding the helix is perforated. The electric ignition assembly is a 0.00l9 in. diameter /20 Ni/Cr bridge-wire surrounded by a bead of a double salt of lead nitrate and lead salt of dinitro-o-cresol/ KClO Se ignition composition and attached to lead wires which extended through the closure plug to an external firing circuit. I

Five of the units are initiated in air at atmospheric pressure; however, initiation of the base charge does not occur even though the donor charge functions properly in all five cases. When five of the units are fired in water at a depth equivalent to 1000 p.s.i. pressure, the initiators function properly and the base charge detonates in all five instances.

We claim:

1. An arming system comprising a donor charge, an acceptor charge separated from the donor charge by distance sufficient to preclude sympathetic actuation of the acceptor charge by a detonation stimulus from the donor charge, and a safe-arm mechanism connecting said charges.

said safe-arm mechanism comprising an exposed helix of 3 to 10 tightly coiled turns of low-energy connecting cord having a core of hi h velocity, cap-sensitive detonating explosive at a loading of about from 1.5 to 2.2 grains per foot encased in a ductile metal sheath, said helix being characterized by its ability to propagate a detonation stimulus when surrounded by a liquid medium and its inability to propagate such a stimulus when surounded by a gaseous medium.

2. An arming system of claim 1 wherein the metal sheath of adjacent turns of the helix is in contact, the ratio of said core loading in grains/ foot to the thickness of said sheath in inches is about from to 220:1, and the internal diameter of the helix is about from 1.75 to 8 times the diameter of the cord.

3. An arming system of claim 2 wherein said cord after the last turn at the end of the helix nearest the acceptor charge extends at least partially across the end of the central opening through said helix.

4. An arming system of claim 2 wherein the ductile metal sheath is lead.

5. An arming system of claim 1 wherein the distance between the donor and acceptor charges is sufficient to preclude and said sympathetic actuation in both air and water, and said gaseous medium is air and said liquid medium is water.

6. An arming system of claim 1 wherein said cord consists essentially of a core of PETN at a loading of about 1.8 gr./ft. encased in a lead sheath having an outer diameter of 0.040 to 0.045 inch, and said helix has 3 to 5 turns of said cord and an internal diameter of about inch, the lead sheath of adjacent turns being in contact.

7. An arming system of claim 6 wherein said cord, after the last turn at the end of the helix nearest the acceptor charge, extends substantially diametrically across the central opening through the helix before it connects to said acceptor charge.

References Cited by the Examiner UNITED STATES PATENTS 3,095,812 7/1963 Coursen 10227 3,099,215 7/ 1963 Brockway et a1 10227 X 3,112,699 12/1963 Noddin 102-20 X 3,169,481 2/1965 Stresau et al. 10227 BENJAMIN A. BORCHELT, Primary Examiner. V. R. PENDEGRASS, Assistant Examiner. 

1. AN ARMING SYSTEM COMPRISING A DONOR CHARGE, AN ACCEPTOR CHARGE SEPARATED FROM THE DONOR CHARGE BY DISTANCE SUFFICIENT TO PRECLUDE SYMPATHETIC ACTUATION OF THE ACCEPTOR CHARGE BY A DETONATION STIMULUS FROM THE DONOR CHARGE, AND A SAFE-ARM MECHANISM CONNECTING SAID CHARGES, SAID SAFE-ARM MECHANISM COMPRISING AN EXPOSED HELIX OF 3 TO 10 TIGHTLY COILED TURNS OF LOW-ENERGY CONNECTING CORD HAVING A CORE OF HIGH VELOCITY, CAP-SENSITIVE DETONATING EXPLOSIVE AT A LOADING OF ABOUT FROM 1.5 TO 2.2 GRAINS PER FOOT ENCASED IN A DUCTILE METAL SHEATH, SAID HELIX BEING CHARACTERIZED BY ITS ABILITY TO PROPAGATE A DETONATION STIMULUS WHEN SURROUNDED BY A LIQUID MEDIUM AND ITS INABILITY TO PROPAGATE SUCH A STIMULUS WHEN SURROUNDED BY A GASEOUS MEDIUM. 